Numerous attempts have been made to modulate a host's immune system as a means for treating cancers. Such attempts include: active immunotherapy using tumor or tumor antigen containing vaccines or immune active lymphokines; adoptive immunotherapy using a host's peripheral blood or tumor infiltrating lymphocytes expanded in culture and reinjected; passive immunotherapy by administration of monoclonal antibodies; and localized immunotherapy using intralesional administration of agents such as Bacillus Calmette-Guerin (BCG). The most effective of these approaches has been localized therapy with BCG for melanoma metastasis to the skin and superficial bladder cancer. While the mechanism of action of BCG is not completely understood, studies clearly show that successful immunotherapy of this type is associated with recruitment of T cells to the tumor.
Cytokines such as the interleukins are important mediators in cell-mediated immune responses in a host. The cell-mediated immune response ("local immune responses") is produced by thymus derived lymphocytes or T-cells. T-cells detect the presence of invading pathogens through a recognition system referred to as the T-cell antigen receptor. Upon detection of an antigen, T-cells direct the release of multiple T-cell lymphokines including, but not limited to, the interleukin-2 family (IL-2). IL-2 is a T-cell growth factor which promotes the production of many more T-cells sensitive to the particular antigen. This production constitutes a clone of T-cells. The sensitized T-cells attach to cells containing the antigen. T-cells carry out a variety of regulatory and defense functions and play a central role in immunologic responses. When stimulated to produce a cell-mediated immune response, some T-cells respond by acting as killer cells, killing the host's own cells when these have become infected with virus and possibly when they become cancerous and therefore foreign. Some T-cells respond by stimulating B cells while other T-cells respond by suppressing immune responses.
Examples of other interleukins which are mediators in cell-mediated immune responses include interferon-.gamma. (IFN-.gamma.), granulocyte-macrophage colony stimulating factor (GM-CSF), interleukin-4 (IL-4), interleukin-5 (IL-5) and interleukin-12 (IL-12). IFN-.gamma. activates macrophages and enhances expression of immune-reactive antigens on tumor cells. GM-CSF activates macrophages and stimulates macrophage and dendrite cell recruitment and differentiation. IL-4 is a T cell derived helper lymphokine which participates in the regulation of growth and differentiation of B and T cells. IL-5 is a T cell derived lymphokine which has its primary effects on the expansion of eosinophils. There is evidence which suggests that eosinophils, when recruited to a tumor site, may have direct anti-tumor effects. IL-12 is a heterodimeric lymphokine initially purified from the conditioned medium of a human B lymphoblastoid cell line. Murine IL-12 has now been cloned and expressed. IL-12 stimulation has been shown to enhance antigen presentation and the cytolytic activity of natural killer cells.
The value of cytokine-based gene therapy was suggested in preclinical murine studies. Inoculation of mice with experimental tumors transfected with genes for tumor necrosis factor (Asher AL, et al., J. Immunol. 1991 146:3227), interleukin-2 (Fearon ER, et al., Cell 1990 60:397), and IL-4 (Golumbek PT, et al., Science 1991 254:713) resulted in growth and subsequent rejection of the injected tumor. In many cases the mice were shown to generate a systemic anti-tumor response. IL-4 transfected tumors regressed and lead to the regression of admixed non-transfected tumors in mice (Tepper PI, et al., Cell 1989 57:503). This immunotherapy was also effective in nu/nu mice demonstrating a non-T cell component which may contribute to localized therapy. IL-4 transfected RENCA cells have been shown to generate specific T cell immunity to the tumor, and result in elimination of pre-existing non-local tumor growth (Golumbek PT, et al., Science 1991 254:713).
Current approaches to this form of therapy involve the growth and stabile gene modification of tumor cells to produce cytokines, their expansion in vitro, and reinjection into the host. While this type of therapy may be feasible in experimental systems, the lack of ability to grow the majority of tumors in vitro, the requirements for in vitro genetic modification of each patient's tumor, and the reinjection of viable tumor into the patient limit the clinical applicability of the approach.
It has now been found that expression of immune active cytokines in tumors can be induced in situ by administration of a vaccinia virus vector. These vaccinia virus vectors can be administered to animals suffering from cancer as a treatment. The vaccinia virus vectors of the present invention are also useful in enhancing immunity to parasites and other invading pathogens which alone fail to invoke an effective host immune response.